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Crosswalking National Wetland Inventory attributes to hydrogeomorphic functions and vegetation communities: a pilot study in the Gallatin Valley, Montana


Published 2006
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The Wetland and Riparian Mapping Center of the Montana Natural Heritage Program has recently begun new National Wetland Inventory (NWI) mapping for Montana. The present study was a pilot project to investigate ways to enhance the new mapping through additional classifications and modifiers. Specifically, our goals were to evaluate GIS-based approaches to Hydrogeomorphic (HGM) classification and profiling, conduct an HGM functional assessment of the Gallatin Valley study area based on these classifications and profiles; establish a crosswalk between NWI classifications, HGM classifications, and vegetation associations; determine whether these additional classifications and associations could be efficiently linked with the National Wetland Inventory in future mapping. Using existing NWI and geospatial data for soils, elevation, and hydrology, we developed a dichotomous key that could assign HGM classes to approximately 85% of the wetland polygons in the study area with 90% accuracy, as determined by visual inspection of the wetlands on 1-meter color imagery. The remaining wetland polygons were classified manually. We used a total of 32 HGM classifications, 17 for unaltered wetlands and 15 for altered wetlands. Nearly 85% of the wetland acreage in the study area is classified as Lotic. Lotic wetland acreage is concentrated in the Gallatin River-Gallatin Gateway (47%) and Lower East Gallatin (25%) watersheds, which contain the main Gallatin River and East Gallatin River channels. Smaller proportions are found in the Smith Creek (10%) and Hyalite Creek (9%) watersheds. Terrene wetland acreage is most plentiful in the Lower East Gallatin (41%) and Smith Creek (27%) watersheds. In the study area as a whole, NWI mapping from the 1980s shows less than 8% of the total wetland acreage as being directly altered. However, when wetland numbers are considered, almost 17% of wetlands were altered in some way. Although 85% of all the wetland acreage in the study area is Lotic, 72% of the altered wetland acreage is Terrene. We used Principal Components Analysis to separate the 5th code HUCs into impact categories, evaluating road density, land cover, population density, septic density, average parcel size, percentage of private land ownership, percentage of public ownership and easements, and percentage of public land survey sections with one or more noxious weeds. The watersheds with the highest impact rankings were the Lower East Gallatin, Hyalite Creek, and the Upper East Gallatin. Together, these watersheds hold 87% of Terrene Interfluve Basin wetlands, 71% of Terrene Interfluve Flat wetlands, and 33% of Terrene Basin wetlands. Because these types of wetlands have high function ratings for nutrient cycling, sediment trapping, aquatic and terrestrial habitat, and wetland biodiversity, we consider those functions to be at risk both within those watersheds and across the study area as a whole. Similarly, those three watersheds contain almost 40% of Lotic River Lower Gradient Floodplain wetlands, 36% of Lotic River Lower Gradient Fringe Wetlands, and 39% of Lotic Stream Lower Gradient Floodplain wetlands. Because these wetland types exist in other, somewhat less impacted watersheds (notably the Gallatin River-Gallatin Gateway and Lower Gallatin), their specific functions (surface water detention, stream maintenance, and plant community maintenance) are at least moderately at risk in the study area. Furthermore, because these Lotic types, like the Terrene types, also have high function ratings for sediment trapping and aquatic/terrestrial habitat, those functions may be especially compromised in these three watersheds. From our field-based surveys and the GIS, we linked National Vegetation Classification System vegetation associations, Montana riparian types, and HGM-derived ecological functions to NWI types in the Gallatin Valley, so users could relate NWI classifications to detailed information useful for planning and management. These associations are complex. A specific NWI type typically encompasses several vegetation associations, but the relationship is usually understandable if past disturbances and regional environmental conditions are considered. NWI associations with HGM modifiers and ecological functions are similarly complex with a one-to-many relationship common. We recommend that mapping in new areas continues to associate vegetation types with USFWS types, since the comprehensive, readily available information about these vegetation types will help those seeking to better understand or manage wetlands. Despite our success with a GIS approach to adding HGM attributes to NWI types, we have not evaluated it beyond the study area, and therefore cannot say whether it offers a cost and time savings over individually classifying each polygon by hand. However, we recommend linking functions to NWI types continues in some form. Wetlands are valued (and regulated) because of their functions and associated values; connecting mapping with functions will aid wetland mitigation, restoration, conservation, and management


Volume 2006
Publisher Helena, MT : Montana Natural Heritage Program
Pages 57
Language English
Call number 578.768012
Book contributor Montana State Library
Contributor usage rights See terms
Collection MontanaStateLibrary; americana

Full catalog record MARCXML


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